The present invention relates to a plate positioning and feeding system for a punch comprising: a punch including a plurality of punching tools; a positioning station disposed upstream of the punch and including positioning means for positioning a plate with respect to a pair of orthogonal axes; a first feed device including first gripping means adapted to be displaced at least along the feed direction by first drive means between leading and trailing positions and arranged to grip a plate in the positioning station when it is in its trailing position and to advance the plate to the punch; a second feed device including second gripping means adapted to be displaced at least along the feed direction by second drive means between leading and trailing positions and arranged to grip a plate when it is in its trailing position and to advance the plate to the punch.
Such systems are used to feed plates, for example of steel or aluminum sheet material, to a punch which punches out a predetermined number of blanks from each plate. By means of said feed devices each plate is moved step by step through the punch and is subjected to punching steps while it is stationary.
Various feeding systems have become known and are shown for example in U.S. Pat. No. 4,382,395, German Patent 34 37 642, European Patent Specification 92 113 561.2 and German Patent 38 41 683.
The punching of the plates should be performed such that as little material as possible will remain in the punched plates in order to save material. On the other hand it is to be avoided that the punching tools punch through the edges of the plates. Accordingly, it is imperative that the plates be advanced as precisely as possible. The prior art feed systems do not allow for precise positioning and/or require sophisticated and costly control systems for precise positioning of the plates.
It is an object of the present invention to provide a plate positioning and feeding system for a punch, which allows for precise positioning of the plates while they are punched.
A further object of the present invention is to provide a plate position and feeding system for a punch, which ensures precise feeding of the plates.
A further object of the present invention is to provide a plate positioning and feeding system for a punch, which allows to punch the plates so that residual meshes of minimum material result and punching through the trailing and leading edges of the plates is avoided.
A further object of the present invention is to provide a plate positioning and feeding system for a punch, wherein idle strokes of the punch are avoided.
A plate positioning and feeding system for a punch as defined above includes, according to the present invention, the following features: the punch includes a pair of rows of punching tools, with the rows being spaced along the feed direction of the plates; said first feed device is arranged to advance the plates to a transfer position which is downstream of said positioning station; said second feed device is arranged to grip each plate when it has reached said transfer position, said transfer position is arranged to correspond to the positioning of the plates during a punching stroke of the punch after the punch has performed at least one preceding punching stroke on a plate; said first and second feed devices cooperate so that the leading row of punching tools performs the last punching stroke with respect to a first plate at the same time when the trailing row of punching tools performs the first punching stroke with respect to the following plate.
In the system of the present invention there are two rows of punching tools spaced from each other along the feed direction. The tools in one row may be aligned to the tools of the other row or may be laterally offset thereto. The present invention uses a pair of feed devices of which the first one feeds a plate from a positioning station wherein the plate is precisely positioned, i.e. precisely aligned with respect to reference coordinates. It is important that the second feed device takes over the plate only after the punch has performed two or more punching strokes. In this manner idle strokes of the punch are avoided. However, this requires that the plates are advanced to the punch without any gaps being present between the plates, such that the last punching step with respect to a plate coincides with the first punching step with respect to the following plate.
With a tool arrangement wherein the leading row of tools includes one tool less that the second row of tools, the--precisely positioned--plate may be guided during the whole punching operation by prongs or other gripping means on the side of the plate which has not yet been punched.
When both rows include the same number of tools, the last advancing step of the plate must be performed by a suitable feeding means, for example a roller feed. Since this is only the last step, all necessary corrections can be performed before. This allows to define the last plate position by mechanical means in the punch instead of for example by an unprecise roller feed.
The prongs or gripping means may be mounted to stiff supports and may be moved by spindles so that high speed cycling is possible. A gripping edge is not required because the feed prongs which hold the plate at the end of the machine operation are disposed laterally of the plates.
Immediately after the change-over the first feed device can be returned to the original position so as to grip a following plate in the positioning station. During the return stroke the gripping means are lowered in order to avoid any interference with the following plates entering the positioning station. The length of the path along which the gripping means of the first feed device have to be moved during the return stroke needs hardly be more than the length of a plate. This may be obtained in a sufficiently short time by means of conventional drive means.
Since the gripping means of the second feed device grip the plate only after the first feed device has advanced the plate already for some punching steps in the punch, the return stroke of the gripping means of the second feed device is relatively short. Also in this case a punching operation without any idle strokes of the punch is possible.
Furthermore, it is important that the drive means of the first and second feed devices are controlled by a numerical control. The numerical control enables advance movements which are precise both with respect to time and space so that idle strokes of the punch are avoided even under extremely unfavourable conditions.
It is known that the plates are cut from band stock material by plate scissors. As a result plate length and angularity of the cut edges may vary within the manufacturing tolerances. In this connection the present invention proposes to provide a pair of sensors which are spaced from each other for a predetermined amount transversely to the feed direction and which are arranged to detect the leading and trailing edges of a plate in order to provide corresponding signals.
The sensors allow to determine any deviation of the leading and, respectively, trailing edges from the transverse axis. The deviation as determined is used to compute a correction value for the numerical control. If such deviation is of a value so that punching through one of the edges of the plate cannot be avoided, it is possible to provide a stop signal. As an alternative one punching stroke may be skipped in order to avoid the production of faulty blanks.
When the plate advances from the positioning station to the position for the first punching operation, the leading edge of the plate passes both sensors. The switch-on signals, i.e. the occurrences of the signals by the sensors will be related to the actual position of the feed device. This position may be determined by the measuring system of the numerical control at a resolution of 0.01 mm. This allows to correct the further feed of the plate according to the measuring result.
If the switch-on signals of both sensors for one edge of a plate will be related to each other, any incorrectness of the angularity of the respective plate edge may be determined.
When the punch includes a pair of rows of tools, only the leading row of tools is used during the first punching step with respect to a certain plate. It must be ensured that the tools do not punch through the leading edge of this plate at this punching step; furthermore, the plate must not extend into the punching area of the second row of tools at this time. From this follows that any deviation of the angularity must be below a predetermined value in order to avoid edge punching. If this cannot be avoided the operation of the punch is interrupted, or the first critical punching step is skipped.
Additionally, the leading edge of the plate altogether may be related to the length of the path which the plate has to be moved to reach the tool. This allows to correct the first punching position, independently of the actual length of the plate, so that edge punching is avoided. Correction of the first punching position of the plate in accordance with the distance for which the plate is still to be moved and as determined by the sensors allows maximally to increase the upper limit for deviations of the angularity. It is desired that only few plates exceed the upper limit even with a small remaining web width of for example 0.98 mm. This ensures to optimally use the space present between the rows of tools. If the plates would be moved into a fixed first punching position which cannot be corrected, it would be necessary to provide a greater value of the remaining web width.
The sensors may be spaced transversely to the feed direction for a predetermined amount and may be arranged to sense the leading and trailing edges of a plate in order to provide corresponding signals. The spacing of the sensors is preferably only slightly less than the width of the plates in order to provide for high measuring accuracy. The numerical control stores a desired value of the length of the plates. The numerical control computes an actual value of the plate length from the two sensor signals. The precise actual length of the plate may be determined by sensing the leading and, respectively, trailing edge of the plate and by use of the values stored in the numerical control. This, however, requires extremely sensitive sensors. This is why the present invention suggests to use laser light sensors as the sensors. The actual length of a plate as determined above is compared to the desired value, and the difference between the desired and actual values is used to determine a correction value for correction of the following feeding steps.
The position of the feed device at which the leading edge of the plate is sensed can be related to a fixed predetermined value which is to be expected as the result of the preceding positioning of the plate when the plate is of normal length. Any deviation of this value indicates a deviation of the desired plate length. Within the geometrical possibilities of the tool arrangement the remaining web width of the plate may be used for correcting the total plate length. Determining the plate length in the above described manner requires that the plate in the positioning station has its trailing and lateral edges precisely positioned with respect to reference coordinates.
During the following machining of the plate the same sensors sense the trailing plate edge in the same manner. The evaluation of any deviation of angularity is the same as with the leading edge, however by using switch-off signals. The angularity is monitored, and the actual spacing of the trailing edge of the plate from the tools is sensed and used for correcting the following punching positions and, respectively, feeding steps.
If the plate length is measured before the first punching step, the control of the first feed device must be corrected when a changed plate length has been detected by the sensors and a correction value has been determined. If a second feed device is used which takes over the plate from a first feed device when the plate is already in the punch, the feed length is changed similarly when an actual plate length deviating from a desired value has been detected in order to avoid punchings through the trailing edge of a plate.
Furthermore, a stop signal may be generated also when the comparison of the plate length desired and actual values shows a deviation which would result in faulty blanks.
The gripping means of the second feed device may be moved for a particularly short path when they are arranged to grip the plate in the transfer position at the trailing end of the plate from opposite sides thereof.
Before the gripping means of the first feed device engage a plate, the plate must be positioned precisely with respect to reference coordinates. This is obtained in the positioning station. Of course, the plate must be moved into the positioning station by suitable conveyor means. To this end, a third feed device is provided, which feeds the plate from a loading station to the positioning station. According to an embodiment of the invention the third feed device includes a pair of parallel conveyor means, for example conveyor bands, which are driven by separate numerically controlled third drive means and which each include a drive member. The drive members are disposed so that one of said drive members is upstream of a plate and the other one is downstream of the plate. The leading drive member which has moved a plate to the positioning station is initially moved into engagement with the leading edge of the following plate, which may require that it is moved back. The leading drive member along with the trailing drive member which engages the trailing edge of the following plate move the plate into the positioning station. Only when this operation has been terminated, the leading drive member returns along with the returning run of the conveyer band, and the leading drive member now becomes the trailing drive member for feeding a further plate towards the positioning station. The separately driven conveyor means allow for rapid and precise feeding of the plate to the positioning station, with the leading drive members acting as abutments during the breaking phase.
As an alternative the third feed device may comprise numerically controlled feed rollers engaging the plate from above and from below. The pairs of rollers are preferably driven by a single servomotor. A first pair of rollers is positioned downstream of the location of plate separation preferably for less than a plate length and is arranged to decelerate and transfer the plate to a second numerically controlled pair of rollers which conveys the plate into the positioning station. Such pair of rollers stops the plate and then is moved apart preferably for a few millimeters in order not to interfere with the positioning operations.